Clathrate or cage inclusion compounds have been known for about 200 years and their deliberate production for about 60 years. The first examples discovered were clathrate hydrates, i.e. enclosures of gas molecules (e.g. Cl2) in ice lattices. Today, especially intermetallic clathrates, known since the 1960s, are subject to extensive research because of their high application potential, particularly in thermoelectric modules. See for example U.S. Pat. No. 5,800,794A1, U.S. Pat. No. 6,188,011B1 and U.S. Pat. No. 6,525,260B2. Intermetallic clathrates are classified into 9 different structural types, of which type 1 has been investigated most extensively, which is why numerous representatives thereof are known. FIG. 1 is a schematic representation of the structure of such a clathrate compound, wherein the larger spheres in the right half of the drawing represent the atoms encapsulated in the cage.
Regarding the term “clathrate”, it should be noted that herein its definition includes mainly compounds in which the cage-forming atoms provide a space-filling network, although the definition of “clathrate” may sometimes, especially in English, comprise all sorts of inclusion compounds.
Components used for intermetallic clathrates may be various combinations of elements. EP 1,074,512 A1, for example, which largely corresponds to U.S. Pat. No. 6,461,581B1, discloses cage inclusion compounds wherein one or more elements from group 4A, especially C or Si, form the matrix, i.e. the “cage”, together with so-called “substitution atoms”, which may be selected from almost all other elements of the periodic table—except for hydrogen, technetium, the rare gases and the transuranium elements. In the above documents, the enclosable atoms may also be selected within a broad range: atoms from groups 1A to 3A as well as those of transition elements—except for group 7B (Mn, Tc, Re) and the transuranium elements—may be enclosed in the matrix. Generally, at the moment it is preferred to use atoms of groups 1A and 2A as inclusion components and, for example, Ga, Ge and Si as cage components.
The production of such clathrates is usually effected by melting down the elements, which, after cooling the mixture to below the melting range, usually results in various phases which either do not yet comprise the desired clathrate compounds at all or only comprise too small proportions of the desired solid clathrate compound in combination with undesired, interfering foreign phases. Thus, in order to obtain phase-pure clathrate compounds, a subsequent heat treatment at temperatures of several hundred degrees Celsius is necessary, usually for a period of several days or even weeks, until a substantially pure, solid phase of the desired clathrate is obtained.
One variation of these methods includes hot compression molding or discharge plasma sintering of powders of the starting materials at about 700° C., which may accelerate the production method (see, for example, U.S. Pat. No. 6,525,260B2).
Nevertheless, in addition to the disadvantage of being very time-consuming, all known methods also require large amounts of energy for reaching and maintaining the necessary temperatures of several hundred degrees Celsius during heat treatment.
Furthermore, the clathrate compounds thus obtained usually are in the form of blocks or compact masses and have to be mechanically processed by laborious methods for later use. Actually, thin layers of clathrate compounds have also been produced. However, this requires laborious pulsed laser ablation or helicon magnetron sputtering methods, and the thickness of the resulting layers measures only several nanometers.
Therefore, a method would be desirable, which allows a) the production of clathrate compounds with layer thicknesses in the micrometer range within a short period of time and with relatively low energy demand, b) the production of clathrates directly on a workpiece or article to be coated therewith, so that such articles may be produced quickly and inexpensively, and c) the production of clathrates with higher purity and improved properties.